Sunday, July 13, 2014

During the second day of Calligra sprint in Deventer we split into two small groups. Friedrich, Thorsten, Jigar and Jaroslaw were discussing global Calligra issues, while Boud and me concentrated on the performance of Krita and its memory consumption.

We tried to find out why Krita is not fast enough for painting with big brushes on huge images. For our tests we created a two-layer image 8k by 8k pixels (which is 3x256 MiB (2 layers + projection)) and started to paint with 1k by 1k pixels brush. Just to compare, SAI Painting Tool simply forbids creating images more than 5k by 5k pixels and brushes more than 500 pixels wide. And during these tests we found out a really interesting thing...

I guess everyone has at least once read about custom memory management in C++. All these custom new/delete operators, pool allocators usually seem so "geekish" and for "really special purposes only". To tell you the truth, I though I would never need to use them in my life, because standard library allocators "should be enough for everyone". Well, until curious things started to happen...

Well, the first sign of the problems appeared quite long ago. People started to complain that according to system monitor tools (like 'top') Krita ate quite much memory. We could never reproduce it. And what's more 'massif' and internal tile counters always showed we have no memory leaks. We used exactly the number of tiles we needed to store the image of a particular size.

But while making these 8k-image tests, we started to notice that although the number of tiles doesn't grow, the memory reported by 'top' grows quite significantly. Instead of occupying usual 1.3 GiB, which such image would need (layers data + about 400MiB for brushes and textures) reported memory grew up to 3 GiB and higher until OOM Killer woke up and killed Krita. This gave us a clear evidence that we have some problems with fragmentation.

Indeed, during every stoke we have to create about 15000(!) 16KiB objects (tiles). It is quite probable that after a couple of strokes the memory becomes rather fragmented. So we decided to try boost::pool for allocation of these chunks... and it worked! Instead of growing the memory footprint stabilized on 1.3GiB. And that is not counting the fact that boost::pool doesn't free the free'd memory until destruction or explicit purging [0]

Now this new memory management code is already in master! According to some synthetic tests, the painting should become a bit fasted. Not speaking about the much smaller memory usage.

Conclusion:

If you see unusually high memory consumption in your application, and the results measured by massif significantly differ from what you see in 'top', you probably have some fragmentation problem. To proof it, try not to return the memory back to the system, but reuse it. The consumption might fall significantly, especially is you allocate memory in different threads.

[0] - You can release unused memory by explicitly calling release_memory(), but 1) the pool must be ordered, which is worse performance; 2) the release_memory() operation takes about 20-30 seconds(!), so there is no use of it for us.

Last weekend we had a really nice sprint Deventer, which was hosted by Irina and Boudewijn (thank you very much!). We spent two days on discussions, planning, coding and profiling our software, which had many fruitful results.

On Saturday we were mostly talking and discussing our current problems, like porting Calligra to Qt5 and splitting libraries more sanely (e.g. we shouldn't demand mobile applications compile and link QWidget-based libraries). Although these problems are quite important, I will not describe them now (the other people will blog about it very soon). Instead I'm going to tell you about a different problem we also discussed — translations.

The point is, when using i18n() macro, it is quite easy to make mistakes which will make translator's life a disaster, so we decided to make a set of rules of thumb which developers should follow for not creating such issues. Here are these five short rules:

Avoid passing a localized string into a i18n macro

Add context to your strings

Undo commands must have (qtundo-format) context

Use capitalization properly

Beware of sticky strings

Next we will talk about each of the rules in details:

1. Avoid passing a localized string into a i18n macro

They might be not compatible in case, gender or anything else you have no idea about

Example 1

Such string concatenation is correct in English, but it is completely inappropriate in many languages in which a noun can change its form depending on the case. The problem is that in macro i18n(“Mask”) the word "Mask" is used in nominative case (is a subject), but in expression "Delete Mask” it is in accusative case (is an object). For example is Russan the two strings will be different and the translator will not be able to solve the issue easily.

Example 2

This case is more complicated. Both words "Monday" and "Friday" are used in the nominative case, so they will not change their form. But "Monday" and "Friday" have different gender in Russian, so the adjective "Last" must change its form depending on the second word used. Therefore we need to separate strings for the two terms.

The tricky thing here is that we have 7 days in a week, so ideally we should have 7 separate strings for "Last ...", 7 more strings for "Next ..." and so on.

Just imagine how many objects can be stored inside the registry. It can be a dozen, a hundred or a thousand of objects. We cannot control the case, gender and form of each object in the list (can we?). The easiest approach here is to put the object name in quotes and "cite" that literally. This will hide the problem in most of the languages.

4. Use capitalization properly

5. Beware of sticky strings

When the same string without a context is reused in different places (and especially in different files), doublecheck whether it is appropriate.

E.g. i18n("Duplicate") can be either a brush engine name (noun) or a menu action for cloning a layer (verb). Obviously enough not all the languages have the same form of a word for both verb and noun meanings. Such strings must be split by assigning them different contexts.

Alexander Potashev has created a special python script that can iterate through all the strings in a .po file and report all the sticky strings in a convenient format.

Conclusion

Of course all these rules are only recommendation. They all have exceptions and limitations, but following them in the most trivial cases will make the life of translators much easier.

In the next part of my notes from the sprint I will write how Boud and me were hunting down memory fragmentation problems in Krita on Sunday... :)

Wednesday, June 18, 2014

Our Kickstarter project is over 50% of the goal now! To say "Thank you!" to our supporters we decided to implement another small feature in Krita: editing of the global selection. Now you can view, transform, paint and do whatever you like on any global selection!

To activate the feature, just go tomain menu and activate Selection->Show Global Selection Mask check box. Now you will see your global selection as a usual mask in the list of layers in the docker. Activate it and do whatever you want with it. You can use any Krita tool to achieve your goal: from trivial Brush tool to complex Transformation or Warp tool. When you are done, just deactivate the check box to save precious space of the Layers docker.

Add usual global selection

Deform with Warp Tool and paint with Brush Tool

This feature (as well as the Isolated Mode one) was a really low hanging fruit for us. It was possible to implement due to a huge refactoring we did in the selections area about two years ago, so adding it was only extending existing functionality. It is really a pity that the other features from the Kickstarter list cannot be implemented so easily :) Now I'm going to dig deep into the Vanishing Points and Transformations problem. Since Saturday I've been trying to struggle through the maths of it, but with limited success...

Monday, June 16, 2014

During the first week of our Kickstarter campaign we collected more than 6500 eur, which is about 43% of the goal. That is quite a good result, so we decided to start implementing the features right now, even though the campaign is not finished yet :)

I started to work on three low-hanging fruits in parallel: perspective transformation of the image basing on vanishing points, editing of the global selection and enhanced isolate layer mode.

It turned out that the vanishing point problem is not so "low-hanging" as I thought in the beginning, so right now I'm in the middle of my way of searching the math solution for it: to provide this functionality to the user the developer must find right perspective transformation matrix basing only on points in the two coordinate systems. This problem is inverse to what everyone is accustomed to solve at school :) This is quite interesting ans challenging task and I'm sure we will solve it soon!

Until I found the solution for maths task, I decided to work on small enhancements of our Isolate Layer mode. We have this feature already for a long time, but it was too complicated to use because the only way to activate it was to select "Isolate Layer" item in the right-click menu. Now this problem is gone! You can just press Alt key and click of the layer or mask in the Layers docker. This enables many great use cases for artists, which now can be done with a single Alt-click:

Show/Edit the contents of a single layer, without other layers interferring

Show/Edit the masks. This includes Selection Masks, so you can edit non-global selections very easily (modifying global selections will be simplified later)

Inspect all the layers you have by simply switching between them and looking into their contents in isolated environment

A comic page by Timothée Giet

There are several more things we are planning to do with the Isolated Mode like adding a bit of optimizations, but that is not for today :)

The next packages in Krita Lime will include this new feature. They are now building at Launchpad and will be available tonight!